In ARC222, held last week, we banned a user who had been in first place at one point during the contest.

This has led to discussion about AtCoder’s ban system, so I would like to explain several points.

General Policy

The people responsible for making ban decisions are well aware that there may be strong competitors who have not participated in many contests before. We do not ban users merely because they achieve good results.

That said, we cannot say that the possibility of a mistaken ban is completely zero. Recently, every time we hold a contest, tens to hundreds of bad actors have attempted to achieve high rankings using AI. Their methods have also become increasingly sophisticated, and we spend a great deal of time and effort making these judgments. As a result, we cannot completely deny the possibility that legitimate participants may be banned by mistake.

However, for users who have already participated in dozens of contests, we conduct thorough investigations and make decisions with particular care.

AtCoder also responds to appeals regarding mistaken bans. In fact, there have been several cases where, after a thorough investigation, a ban decision was overturned.

Why We Do Not Publicize Submissions from Banned Users

There have been requests for us to make submissions from banned users public, but we keep them private for the following reasons:

• Analyzing submissions from banned users could make it possible to infer AtCoder’s ban criteria.
• Due to our Privacy Policy, we cannot display code from deleted accounts.

Regarding This Specific Case

In the following, I will refer to the user at the center of this discussion as X.

As a rule, we do not provide detailed explanations regarding individual bans. However, X is a user who had earned a certain degree of trust within the community, and we have received multiple comments suggesting that this may have been a mistaken ban by AtCoder. For that reason, we will make an exception and explain this case.

I have reviewed this ban. Based on that review, I believe the ban decision was not mistaken.

The reasons are as follows:

• X’s submission to Problem F was written using a template and style completely different from X’s other submissions. This was not limited to one part of the code; the difference was apparent throughout the entire code.
• X deleted their own account three minutes after submitting to Problem F, and before AtCoder carried out the ban.

Normally, once we have conducted an internal review and confirmed the validity of a ban, our handling of an appeal regarding a mistaken ban is complete. However, I believe that this alone does not sufficiently answer the questions that have been raised about AtCoder’s handling of this matter. In particular, I understand that there is some merit to the opinion that AtCoder should disclose the code in question.

Therefore, if X claims that this was a mistaken ban, we ask X to take the following actions:

• Allow AtCoder to make past submissions visible again.
• Explain why the submission to Problem F in ARC222 was written in a style completely different from the submissions to the other problems.
• Explain why X deleted their own account three minutes after submitting to Problem F.

As explained above, we generally do not publicize submissions from banned users. However, in this case, the code is so different that it is clear it was not written by X themselves, so we believe there is little concern that publishing it would allow others to infer our ban criteria.

Therefore, if we receive clear permission from X, we can show the code and answer the community’s questions.

Conversely, if X does not claim that this was a mistaken ban, we ask those around X to refrain from asserting and spreading the claim that AtCoder’s decision was a mistaken ban.

To the best of my knowledge, we have not received any appeal from X themselves at this time.

The purpose of this statement is not to further criticize X. Its purpose is to prevent inaccurate rumors about AtCoder from spreading, and to provide the necessary explanation regarding our ban system.

Again, this is an exceptional response. Please refrain from discussing bans of other users.